The objectives of this research are to investigate the mechanisms by which serum proteins are transported into urine in the normal state, and in the naturally occurring and experimental disease states. The amount of albumin excreted in rats with mannitol diuresis, coupled with measurements of GFR, UNa, and V, will be used to assess the effect of decreased fractional reabsorption on protein reabsorption. Tubular poisons will be examined as an approach to measuring the sieving coefficient of proteins at the glomerular basement membrane. A study of the comparative excretions of a series of albumins and albumin derivatives in the normal and tubular poisoned rats should make it possible to isolate the factors influencing selective reabsorption from those responsible for glomerular permeability.